1. Field of the Invention
The present invention relates to a plant control system for controlling a plant.
2. Description of the Prior Art
The inventors already proposed a control system for controlling the air-fuel ratio of an air-fuel mixture to be combusted by an internal combustion engine to achieve optimum purifying performance of a catalytic converter such as a three-way catalytic converter disposed in the exhaust passage of the internal combustion engine, as disclosed in Japanese laid-open patent publication No. 9-273438 which corresponds to U. S. patent application No. 08/835192 now U.S. Pat. No. 5,924,281.
According to the proposed control system, the air-fuel ratio of the internal combustion engine is controlled to adjust the concentration of a certain component, e.g., oxygen, in an exhaust gas that has passed through the catalytic converter highly accurately to a predetermined optimum value for thereby keeping the catalytic converter maximally effective to purify the exhaust gas regardless of aging thereof. The control process is carried out as follows:
The control system has an O.sub.2 sensor disposed downstream of the catalytic converter for detecting the concentration of oxygen contained in the exhaust gas which has passed through the catalytic converter. The control system sequentially determines a manipulated variable for determining the air-fuel ratio of the internal combustion engine, specifically, a target air-fuel ratio for the exhaust gas, such that an output of the O.sub.2 sensor will be of a given target value corresponding to the above predetermined optimum value for the oxygen concentration. The control system then controls the air-fuel ratio of the internal combustion engine according to the target air-fuel ratio, more specifically, controls the amount of a fuel supplied to the internal combustion engine so that the air-fuel ratio of the air-fuel mixture to be combusted by the internal combustion engine will be equalized to a target air-fuel ratio. In controlling the air-fuel ratio of the internal combustion engine according to the target air-fuel ratio, the air-fuel ratio of the internal combustion engine, more accurately, the air-fuel ratio of the air-fuel mixture from which the exhaust gas entering the catalytic converter is produced by combustion, is detected by an air-fuel ratio sensor disposed upstream of the catalytic converter, and the air-fuel ratio of the internal combustion engine is controlled by a feedback loop so that the detected air-fuel ratio will be equalized to the target air-fuel ratio.
In the above control system, the exhaust system, including the catalytic converter, which ranges from a position upstream of the catalytic converter to a position downstream of the catalytic converter, may be considered to be a plant for generating and emitting an exhaust gas having an oxygen concentration detected by the O.sub.2 sensor, from an exhaust gas having an air-fuel ratio detected by the air-fuel ratio sensor. The internal combustion engine may be considered to be an actuator for generating and emitting an exhaust gas having an air-fuel ratio to be supplied to the plant. Thus, the above control system proposed by the inventors may be expressed as a system for determining a target input for the plant (more generally, a manipulated variable which defines an input to the plant) such that an output from the O.sub.2 sensor (an oxygen concentration of the exhaust gas) as an output from the plant will be equalized to a given target value, and controlling an output of the internal combustion engine (=an input to the plant) as the actuator according to the target input.
According to the above control system, it is necessary to control the output from the O.sub.2 sensor highly accurately at the target value for the purpose of attaining the optimum purifying performance of the catalytic converter. However, the exhaust system including the catalytic converter has a relatively long dead time, i.e., a period of time required until the air-fuel ratio corresponding to the exhaust gas entering the catalytic converter will be reflected in the output from the O.sub.2 sensor disposed downstream of the catalytic converter, and the dead time tends to adversely affect the control process of the control system, i.e., tends to reduce the stability and quick response of the convergence of the output from the O.sub.2 sensor toward the target value. In view of the above difficulties, an output of the O.sub.2 sensor, i.e., a future value of the output of the O.sub.2 sensor, after the dead time of the exhaust system is sequentially estimated based on the output from the O.sub.2 sensor and the output from the air-fuel ratio sensor. Using the estimated output, the target air-fuel ratio is determined to compensate for the dead time of the exhaust system. Therefore, the target air-fuel ratio is made highly reliable, and the controllability of the output from the O.sub.2 sensor at the target value, i.e., the stability and quick response of the control process, is increased.
A further study conducted by the inventors has revealed that for increasing the controllability of the output from the O.sub.2 sensor at the target value, it is desirable to compensate for not only the dead time of the exhaust system including the catalytic converter, but also the dead time of the internal combustion engine and an engine controller for controlling the air-fuel ratio of the internal combustion engine based on the target air-fuel ratio. Generally, the internal combustion engine and the engine controller are a system for generating an exhaust gas to enter the catalytic converter from the data of the target air-fuel ratio. The dead time of the internal combustion engine and the engine controller is a period of time required until the data of the target air-fuel ratio will be reflected in the actual air-fuel ratio corresponding to the exhaust gas entering the catalytic converter.
Specifically, the dead time of the internal combustion engine and the engine controller is affected by operating conditions of the internal combustion engine. While this dead time may be short enough not to impair the process of controlling the output from the O.sub.2 sensor at the target value, it may also be relatively long depending on operating conditions of the internal combustion engine. If the dead time is relatively long, then the controllability of the output from the O.sub.2 sensor at the target value is lowered.